there is also criticallity and multiplication factor

1. PRESENTATION TOPIC
1. NEUTRON FLUX AND REACTOR POWER
2. CRITICALITY AND MULTIPLICATION FACTOR
ASSIGNED BY
PROF DR. AZIZA AFTAB
GROUP MEMBERS
16CH06
16CH20
16CH25
16CH36
16CH57

2. Neutron flux:
 Neutron flux (φ), defined as the number of neutrons
produced per unit fission, crossing through some arbitrary
cross section unit area in all direction per unit time.
 Its unit is neutron/cm2s.

3. Mathematical Formula for calculating
Neutron flux
Ф = n.v
where:
Ф – neutron flux (neutrons.cm-2.s-1)
n – neutron density (neutrons.cm-3)
v – neutron velocity (cm.s-1)
Neutron density: is the number of neutrons existing in one cubic
centimeter.

4. Device for Measurement of the Neutron flux
 The neutron flux is usually measured by excore neutron
detectors, which belong to so called the excore nuclear
instrumentation system (NIS).
 This system monitors the power level of the reactor by
detecting neutron leakage from the reactor core.

5. Cont….
 The excore nuclear instrumentation system is considered a
safety system, because it provide inputs to the reactor
protection system during startup and power operation.
This system is of the highest importance for reactor
protection system.

6. Reactor power:
Defined as
 Reactor power = Energy released per fission (Ef)* Number
of fissions events per second (fission/s)
 Its unit is MeV.s-1

7. Formula for calculating Reactor power
P = Ф . NU235 . σf
235 . Er . V
where
P – reactor power (MeV.s-1)
Ф – neutron flux (neutrons.cm-2.s-1)
σ – microscopic cross section (cm2)
N – atomic number density (atoms.cm-3)
Er – the average recoverable energy per fission (MeV / fission)
V – total volume of the core (m3)

8. Types of Reactor Power
 There are three types of reactor power outputs from any
nuclear reactor.
1. Nuclear Power
2. Thermal Power
3. Electrical Power

9. 1. Nuclear power:
 The rate at which nuclear reactions release nuclear energy
to generate heat, which most frequently is then used in
steam turbines to produce electricity in a nuclear power
plant.
 Nuclear power can be obtained from nuclear fission,
nuclear decay and nuclear fusion reactions

10. 2. Thermal Power
Defined as
 The rate at which heat is produced in the reactor core as the result
of fissions in the fuel.
 The amount of energy released per one fission reaction about 200 MeV/fission.
 If we want to know the reaction rate or thermal power of the reactor core,
it is necessary to know how many neutrons are traveling through the
material.

11. 3. Electrical Power
 Electric power is the rate at which electrical energy is
generated by the generator.
 For example, for a typical nuclear reactor with a thermal
power of 3000 MWth, about ~1000MWe of electrical
power is generated in the generator.

12. Pictorial view of reactor power in nuclear
reactor

13. Example problem

14. Criticality and Multiplication factor
 When a nuclear chain reaction proceeds in a nuclear reactor there are some condition
at which this reaction starts and proceed further,
Condition 1. Criticality factor and multiplication factor
 A criticality factor or effective multiplication factor (k) is defined to denote the relative
number of neutrons produced in successive fission events.
 ‘k’ is the ratio of number of neutrons produced by fission in one generation to the
number of neutrons produced by fission in the preceding generation.

15. Cont…
 If a system is critical, the multiplication factor is 1.
 Supercritical systems have multiplication factors greater
than 1, while subcritical systems have multiplication
factors lesser than 1.

16. Condition 2. Critical mass
 Such a mass of uranium in which one neutron out of all the
neutrons produces further fission is called critical mass.
 Note : if the mass of uranium is much greater than the
critical mass then chain reaction proceeds at a rapid speed
and huge explosion is produced. Atom bomb works at this
principle.

17. Cont…
 If the mass of uranium is less than the critical mass the
chain reaction does not proceed.
 If the mass of uranium is equal to critical mass, the chain
reaction proceeds at its initial speed and in this way we
get a source of energy.
 Energy in atomic reactor is obtained according to this
principle.